JM, You got it... Just use what you have and split it into 4.
I hear what you are saying about the old AM/FM in the dash... I would do the same but the stereo I have in the dash is custom fit and really nice. I'm not tired of it yet. When I do get tired of it I'm going with an old style AM/FM cassette. The adaptor for the cassette to MP3 is awesome. The sound is perfect. The AM/FM convertors suck!